In recent years, as countermeasures for air pollution and global warming, it has been eagerly desired to reduce carbon oxide emissions. In the auto industry, introduction of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been expected to reduce carbon oxide emissions, and secondary batteries for driving motors, which are key devices in putting EVs and HEVs into practical use, are being actively developed. As the secondary batteries for driving motors, lithium ion secondary batteries having high theoretical energy are attracting attention and are now being developed rapidly. However, lithium is not in more abundant supply than sodium, for example, and is expensive. Accordingly, sodium ion secondary batteries are currently being developed for the purposes of achieving reduction of cost and stable supply of batteries.
Conventionally-proposed electrolytic solutions for sodium ion batteries are capable of including as the positive-electrode active material, substances which have low viscosity and high specific electric conductivity; are hardly decomposed even with high electrical potential; and are charged and discharged in the region of high positive electrical potential. For example, proposed is an electrolytic solution for sodium ion batteries that contains one of a chain saturated hydrocarbon dinitrile compound and a nitrile compound such as a chain cyanoether compound and cyanoacetate ester, and at least one of a cyclic carbonate, a cyclic ester, and a chain carbonate (see PTL 1).